One of my most popular posts is [*How to block and hide Facebook applications and requests*](/journal/2008/01/26/how-to-block-and-hide-facebook-applications/). The post is really old and is probably no longer useful, but I do get a lot of requests from people who want to know how to unblock a Facebook application (e.g. Farmville, Cafe World) once it has been blocked. Here's how you do it -- at least, until Facebook makes another privacy interface change.
First, log into Facebook and go to **Account->Privacy Settings**.

Look at the bottom of the page for **Block Lists** and click on **Edit your lists**.

In the **Block Lists**, you can manage all of the people and applications you have blocked. I always block people I never I want to have anything to do with (in **Block users**), but I also block application invites from friends who are app happy (who invite you to every stupid quiz app and game in Facebook land). If I get more than one app invite from someone, I block them.

If you want to re-enable an app you have previously blocked, you'll find the app in the **Blocked applications** area. Click on **Unblock** next to the app in question, and you're all set. Just don't invite me to be a user. :)